Abstract: The yeast cytochrome bc(1) complex, a component of the mitochondrial respiratory chain, is composed of ten distinct protein subunits. In the assembly of the bc(1) complex some ancillary proteins, such as the chaperone Bcs1p, are actively involved. The deletion of the nuclear gene encoding this chaperone caused the arrest of the bc(1) assembly and the formation of a functionally inactive bc(1) core structure of about 500kDa. This immature bc(1) core structure could represent, on the one hand, a true assembly intermediate or, on the other hand, a degradation product and/or an incorrect product of assembly. The experiments here reported show that the gradual expression of Bcs1p in the yeast strain lacking this protein was progressively able to rescue the bc(1) core structure leading to the formation of the functional homo-dimeric bc(1) complex. Following Bcs1p expression, the mature bc(1) complex was also progressively converted into two super-complexes with the cytochrome c oxidase complex. The capability of restoring the bc(1) complex and the super-complexes was also possessed by the mutated yeast R81C Bcsp1. Notably, in the human ortholog BCS1L the corresponding point mutation (R45C) was instead the cause of a severe bc(1) complex deficiency. Differently from the yeast R81C Bcs1p, two other mutated Bcs1p's (K192P and F401I) were unable to recover the bc(1) core structure in yeast. This study identifies for the first time a productive assembly intermediate of the yeast bc(1) complex and gives new insights into the molecular mechanisms involved in the last steps of bc(1) assembly.CI - Copyright (c) 2010. Published by Elsevier B.V.